The present invention relates to a driving force control unit for a vehicle, which is capable of switching driving force to be transmitted to driving wheels in accordance with depression of the brake pedal.
In the prior art, there is known a vehicle in which driving force is transmitted to the driving wheels when the transmission is selected to a running range and the vehicle is in idling condition at a certain or lower vehicle speed including the condition that the vehicle stops. Such driving force is called xe2x80x9ccreep forcexe2x80x9d, by which unintentional backward displacement of the vehicle on a slope is restricted or an improvement is made on the run at a traffic jam. In this conventional vehicle, creep force is generated even when the brake pedal is depressed in the idling condition at the certain or lower vehicle speed. To this end, there is a drawback in that such a vehicle requires strong brake pedal depression for stopping, compared to a vehicle without generating creep force. Further, since the creep force generated by the rotation of the engine is forcibly restricted by braking force, the vehicle is susceptible to vibrations or noise.
In order to solve this problem, Japanese Patent Publication No. 1-244930 (i.e., Japanese Patent Application No. 63-71520) discloses a control unit for a vehicular automatic clutch, which is applied to a control system for generating low dragging torque (creep force) when the transmission is selected to a running range and the vehicle moves at an extremely low speed. The control unit makes creep force lower at a depression of the brake pedal than at a release of the brake pedal. According to this disclosure, creep force is changed by the depression of the brake pedal from a high state (strong creep condition) to a low state (weak creep condition) so that the aforementioned problems, such as the strong brake pedal depression requirement and the vibrations during stops of the vehicle can be eliminated.
In Japanese Patent Publication No. 9-202159 (i.e., Japanese Patent Application No. 8-12457), there is disclosed a vehicle with a starting clutch, in which the starting clutch is half engaged so as to apply the vehicle with driving force (creep force) when the transmission is selected to a running range and the vehicle moves at an extremely low speed. In this vehicle, engaging state of the starting clutch is controlled in response to a braking operation of the driver so that driving force is made lower (in the weak creep condition) at a depression of the brake pedal than at a release of the brake pedal.
However, when creep force is decreased from the strong creep condition to the weak creep condition in accordance with depression of the brake pedal, driving force to be transmitted to the driving wheels lowers. The driver recognizes the reduction of the driving force as braking force. Therefore, if the brake pedal is depressed when the vehicle moves at a certain or lower vehicle speed with the engine being in idling condition, the vehicle receives braking force by the reduction of creep force as well as by the depression of the brake pedal. As a result, the driver feels unintentional deceleration of the vehicle.
Especially in the prior art vehicle, the difference of the creep force (difference of the driving force values) between the strong creep condition and the weak creep condition is set greater, in order to achieve various purposes such as restriction of the vehicle in the strong creep condition from unintentional backward displacement on a slope, noise reduction of the vehicle in the weak creep condition upon depression of the brake pedal, and the like. For this reason, when the driver depresses the brake pedal, he receives unintentional strong deceleration, which is more than the actual brake pedal depression and is recognized as an awkward feel.
With the foregoing drawback of the prior art in view, the present invention seeks to provide a driving force control unit for a vehicle, which prevents the driver""s unintentional strong deceleration feel.
According to the present invention, there is provided a driving force control unit for a vehicle, which allows transmission of driving force from a driving motor to driving wheels irrespective of releasing an accelerator pedal at a certain or lower vehicle speed when a transmission is selected to a running range, and which switches the magnitude of the driving force while the accelerator pedal is released at a vehicle speed no more than the certain vehicle speed between a greater condition and a smaller condition in accordance with depression of a brake pedal so that the driving force is made lower at a depression of the brake pedal than at a release of the brake pedal, wherein the driving force value in said greater condition at a certain or lower vehicle speed is changed according to the vehicle speed, and the change of the driving force value is characterized in that the driving force value becomes smaller as reaching from the vehicle speed at the maximum driving force value to said certain vehicle speed, and further switching said driving force, before stopping the vehicle, from said greater condition to said smaller condition is permitted merely in the vicinity of said certain vehicle speed.
Also, there is provided a driving force control unit for a vehicle, which allows transmission of driving force from a driving motor to driving wheels irrespective of releasing an accelerator pedal at a certain or lower vehicle speed when a transmission is selected to a running range, and which switches the magnitude of the driving force while the accelerator pedal is released at a vehicle speed no more than the certain vehicle speed between a greater condition and a smaller condition in accordance with depression of a brake pedal so that the driving force is made lower at a depression of the brake pedal than at a release of the brake pedal, wherein the driving force control unit further comprises means for changing the driving force value in said greater condition at a certain or lower vehicle speed so that the driving force value is changed according to the vehicle speed, the change of the driving force value being characterized in that the driving force value becomes smaller as reaching from the vehicle speed at the maximum driving force value to said certain vehicle speed, and switching said driving force, before stopping the vehicle, from said greater condition to said smaller condition being permitted merely in the vicinity of said certain vehicle speed.
In such driving force control units, reduction of the driving force upon depressing the brake pedal is carried out only when the difference between the driving force values before and after decreasing the driving force becomes smaller.
Here, the term xe2x80x9ca certain vehicle speedxe2x80x9d indicates a vehicle speed just before the vehicle stops. Therefore, if a certain vehicle speed is 5 km/h as an example shown in the preferred embodiments and examples, the vehicle speed range xe2x80x9cat a certain or lower vehicle speedxe2x80x9d includes vehicle speeds from 0 km/h (when the vehicle stops) to 5 km/h.
Meanwhile, the xe2x80x9csmaller conditionxe2x80x9d indicates a weak creep condition. However, the xe2x80x9csmaller conditionxe2x80x9d includes not only a case that the absolute value of the driving force generated by the driving motor is made smaller, but also a case that the driving force to be transmitted to the driving wheels becomes zero by completely reducing the engaging force of a hydraulically engaging element such as a starting clutch.
The xe2x80x9cvehicle speedxe2x80x9d in the wording xe2x80x9cchanged according to the vehicle speedxe2x80x9d includes vehicle speed itself and its equivalent parameters. For example, as shown in the preferred embodiments and examples, if the vehicle speed and the speed ratio of the starting clutch (speed ratio between the input and output sides of the starting clutch) are in corresponding relation, the driving force may be changed in accordance with the speed ratio. This is also included in the case xe2x80x9cchanged according to the vehicle speedxe2x80x9d.
Further, the term xe2x80x9cvicinityxe2x80x9d appeared in the wording xe2x80x9cpermitted merely in the vicinity of said certain vehicle speedxe2x80x9d indicates a vehicle speed range from a certain vehicle speed to the vehicle speed approximately a half extent of the maximum driving force value, and xe2x80x9ca certain vehicle speedxe2x80x9d itself is also included in its vehicle speed range. The wording xe2x80x9cpermitted merely inxe2x80x9d includes the following three cases: (1) permitting merely in a particular vehicle speed range within the vicinity of the certain vehicle speed; (2) permitting merely at a particular vehicle speed within the vicinity of the certain vehicle speed; and (3) permitting in all the vehicle speed range within the vicinity of the certain vehicle speed.
Here, judgement of the xe2x80x9cvicinityxe2x80x9d (judgement for smaller driving force values) may be carried out not only by the vehicle speed itself, but also by its equivalent parameters. For example, the judgement may be made based on the speed ratio between the input and output sides of the starting clutch or the hydraulic pressure command value controlling the engaging force of the starting clutch (driving force transmission capacity [or driving force value]) in consideration of a characteristic that such a hydraulic pressure command value changes corresponding to the vehicle speed (speed ratio). When a driving force value in each vehicle speed or speed ratio is calculated with the use of coefficients, which are determined and changed corresponding to the vehicle speed or the speed ratio, the judgement may be made based on these coefficients.
An automatic transmission with a fluid type torque converter is widely known in the field, which is made in combination of a fluid type torque converter as a driving force transmission means and a power transfer including a hydraulically engaging element such as a hydraulic clutch and a hydraulic brake. Such a fluid type torque converter is free from external control and has own characteristic in that the driving force value to be transmitted is decreased as the vehicle speed increases (the torque amplification factor is decreased when transmitting an input torque of the engine to the power transfer). In such case, switching between the strong creep condition and the weak creep condition is carried out by switching the engaging force (driving force transmission capacity) of the hydraulically engaging element included in the power transfer into the following two states; (1) complete engaging state (no sliding occurs), and (2) smaller or zero engaging force state (greater sliding). Further, in the strong creep condition, the characteristic of the driving force value to the vehicle speed recited in the claims is obtained without external control. In this case, the vicinity range at a certain or lower vehicle speed is determined in consideration of the torque amplification factor of the fluid type torque converter.
Here, the torque amplification factor of the fluid type torque converter indicates relations between the speed ratio of the fluid type torque converter (which is an index showing the degree of sliding, and is also an equivalent parameter of the vehicle speed in the case that the hydraulically engaging element is in complete engaging state) and the torque amplification factor. As the speed ratio is smaller, i.e., greater sliding and slower vehicle speed, the torque amplification factor becomes higher.